Vehicle emission control systems may be configured to store refueling vapors, running-loss vapors, and diurnal emissions in a fuel vapor canister, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to engine intake for combustion, further improving fuel economy for the vehicle. In a typical canister purge operation, a canister purge valve coupled between the engine intake and the fuel vapor canister is opened, allowing for intake manifold vacuum to be applied to the fuel vapor canister. Fresh air may be drawn through the fuel vapor canister via an open canister vent valve. This configuration facilitates desorption of stored fuel vapors from the adsorbent material in the canister, regenerating the adsorbent material for further fuel vapor adsorption.
However, engine run time in hybrid electric vehicles (HEVs) and plug-in hybrid vehicles may be limited, and thus opportunities for purging fuel vapor from the canister may also be limited. If the vehicle is refueled, saturating the canister with fuel vapor, and then parked in a hot, sunny location prior to a purge event, the canister may desorb fuel vapors as it warms up, leading to bleed emissions. For vehicles that vent the fuel tank during a vehicle-off condition, the volatization of fuel under similar conditions may overwhelm the capacity of the fuel vapor canister. Additionally, under certain conditions, a fuel vapor canister saturated with fuel vapor may desorb fuel vapors during vehicle operation under conditions where the vehicle is being solely powered by a battery. Furthermore, limited engine run times in hybrid and plug-in hybrid vehicles may result in exhaust catalyst temperatures dropping below the light-off range for vehicles relying on exhaust heat to increase the temperature of the catalyst, thus resulting in increased exhaust emissions.
One approach for addressing these problems is described by Robichaux and Kotre in US Patent No. 20020083930 A1. Therein, a method for purging the fuel vapor canister is provided for a HEV comprising commanding the engine to come on during vehicle idle conditions so that the purging process may be executed. By controlling throttle position, sufficient intake manifold vacuum may be provided such that fuel vapor may be rapidly drawn into the engine intake. However, the inventors have herein recognized that the above approach has some issues. For example, turning on the internal combustion engine solely to perform a purge operation may reduce the operating efficiency of the HEV as a result of additional fuel being consumed in order to start the engine. Furthermore, such an approach may not be practical if exhaust heat is relied upon for providing the heat source to increase the temperature of the exhaust catalyst.
Another approach to address the above problems is described by Reddy in U.S. Pat. No. 7,059,306 B2. Therein a method and system is provided for evaporative emission control for a hybrid vehicle using activated carbon fibers. Briefly, fuel vapors from the fuel tank of a hybrid vehicle are first exposed to a quantity of activated carbon granules, and any hydrocarbon vapors not adsorbed by the activated carbon granules (“bleed emissions” or “breakthrough”) are passed through a scrubber containing an activated carbon fiber material capable of adsorbing substantially all of the higher volatility hydrocarbons (e.g., butane, pentane). Implementation of the activated carbon fiber scrubber device serves to decrease emissions, however, the inventors have herein recognized that the above approach additionally has some issues. For example, addition of a scrubber element increases the cost and complexity of the evaporative emissions system, and under certain conditions the scrubber element may be overwhelmed by vapor thus resulting in increased emissions.
The inventors herein have recognized the above issues, and have developed systems and methods to at least partially address them. In one example, a method is provided, comprising, while an engine is off, adsorbing fuel tank vapors in an adsorbent, the vapors generated from a fuel tank, and in response to detecting breakthrough of the vapors from the adsorbent while the engine is off, routing the vapors from the adsorbent through the engine into a catalyst coupled to an exhaust of the engine. For example, routing the vapors from the adsorbent through the engine into a catalyst may include spinning the engine unfueled and stopping the spinning such that both intake and exhaust valves of a first cylinder are configured in an open position, opening a canister purge valve (CPV), closing a throttle, and applying air pressure to the fuel vapor canister. In this way, the fuel vapor canister may be coupled to the exhaust catalyst, and by applying pressurized air to the canister, vapors may be desorbed and routed to the exhaust catalyst while the engine is off.
As one example, a method is provided, comprising, responsive to an indication of vapor breakthrough from the adsorbent while the engine is off, determining whether the temperature of the exhaust catalyst is below a threshold temperature, and if so, electrically heating the exhaust catalyst to a predetermined temperature (e.g., 600° C.). In this way, responsive to an indication of vapor breakthrough during engine-off conditions, a purging event may be commenced when the catalyst is at or above a predetermined temperature such that desorbed vapors routed to the exhaust catalyst are efficiently oxidized, thereby reducing evaporative emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.